Which of these numbers is prime? ${56,\ 65,\ 77,\ 87,\ 97}$
A prime number has exactly two factors: 1 and itself. The factors of 56 are 1, 2, 4, 7, 8, 14, 28, and 56. The factors of 65 are 1, 5, 13, and 65. The factors of 77 are 1, 7, 11, and 77. The factors of 87 are 1, 3, 29, and 87. The factors of 97 are 1 and 97. Thus, 97 is a prime number.